In particular in underground mining a large number of hydraulically actuated roof props or shield support props are required, in order to hold open the underground spaces such as a longwall face or roadway as produced in mining. Furthermore hydraulic operating rams also come into use in the advancing movement of extraction equipment and similar. The shield support props or roof props can be configured as single operating rams or as telescopic multiple operating rams, while the hydraulic rams providing advancing movement are mostly configured as double-acting hydraulic operating rams, which can be actuated by the stroke force in the outward stroke and inward stroke directions. Here each operating ram includes at least one cavity in the interior of a cylinder, and also a piston with piston rod as an axially moving body, each comprising or consisting of at least two parts, which must be connected together so as to be sealed against fluid leakage, and in the connected joint state must be able to withstand high hydraulic pressures of several hundred bar.
Telescopic roof props are known for example from DE-AS 1 2 07 317, DE 100 45 680 A1 or DE 43 23 462. Each ram stage includes a cylindrical tube as a first functional part and a cylinder head cover as a second functional part which are connected together in a sealed manner by means of snap rings, welded connections or screwed connections. Welding offers the advantage that no leakages occur at the connection junctions. The disadvantage with welding is, however, that the structure of the tubes used for the cylinders and the structure of the cylinder head cover, collar rings or base plates can be impaired or destroyed by the process of welding. To be able to use cold-drawn tubes for the cylinders, it was therefore recommended in DE 43 23 462 C2 that the wall sections of the cylinders be secured with the corresponding base plates or cylinder head covers by means of shear wires that are inserted into a connecting groove. Other joining connections between the functional parts of hydraulic or pneumatic operating devices consist of matching threads, pins, collets and similar. The sealing of the separating gap or separation gap between the parts connected together is achieved by means of static seals.